The prior art contains a number of devices which employ a plurality of nozzles directing fluid jets against the surfaces of various products such as food portions to effect rapid heat transfer, either to heat or to cool the product. These devices work by breaking up the boundary layer surrounding the product, thereby increasing the surface heat transfer coefficient. The efficiency of this process is of particular importance in cooling and freezing applications, because inefficiencies manifest themselves in the form of additional heat.
After the fluid jets impinge against the surface of the product, the fluid should be directed to a return plenum in a way that will not permit it to flow horizontally over the surface, since such action tends to re-establish laminar flow. If horizontal flow is permitted, the cumulative horizontal component of fluid velocity becomes greater as the last jet-forming nozzle is approached prior to exit to the return plenum, since the fluid from all preceding nozzles must exit through the zone of the final nozzles. This large horizontal velocity component impedes the action of the vertically impinging jets in breaking through the boundary layer surrounding the product to be treated, and also tends to re-establish laminar flow conditions with their attendant insulating boundary layer effect.
It is the purpose of the present invention to minimize the effects of the boundary layer problem by providing a multiplicity of return paths for fluid flow to the return plenum which do not allow the fluid to establish horizontal or laminar flow across the paths of the impinging jets.